mfortable" construction interface by some methods
  serving as simple shortcuts.

  30.11.2001 Goetz
  Added a class type to irgs.  This class type is to be used as owner
  for local entities of the procedure. This are all entities allocated
  for each execution of the procedure.
  The class type is allocated automatically by new_ir_graph().  Access
  routines to get this type are in irgraph.h.
  We chose a class type as this allows to represent "inner" procedures
  as, e.g., in Pascal.
  Adapted array-stack-example.

[r285]